Electrical circuit for pulse fed inductive load

ABSTRACT

A circuit for applying current pulsations of opposed polarity to an inductive load wherein the net current in the load is the difference in the magnitude of the pulsations. A capacitance is connected across the inductive load to permit the buildup of a net current. Circulating currents in the capacitance can be restricted by inductance or resistance in series between the source of pulsations and the capacitance.

United States Patent [72] Inventor Richard C. Lmhbough Toledo, Ohio [21]App]. No. 767,276 [22] Filed Oct. 14, 1968 Division of Ser. No. 373.136.June 4. 1964. Pat. No. 3,435,916 [45] Patented July 13,1971 [73]Assignee Reliance Electric Company Euclid, Ohio [54] ELECTRICAL CIRCUITFOR PULSE FED INDUCTIVE LOAD 6 Claims, 19 Drawing Figs. [52] U.S.Cl318/158, 318/257, 318/293, 318/300, 318/341 [51] Int. Cl 1102p 5/24,H02p 7/24 [50] Field of Search 318/146, 158, 341, 356, 345, 257, 293,294, 300 [56] References Cited UNITED STATES PATENTS 3,176,212 3/1965DePuy 321/18 3,191,112 6/1965 Cain 321/18 3,219,908 11/1965 Zarleng318/294 UX 3,233,161 2/1966 Sikorra 318/257 3,332,000 7/1967 Greening318/345 3,411,063 11/1968 Schoonover 318/345 3,432,740 3/1969 Morgan321/43 3,170,104 2/1965 Richards.... 318/146 3,184,670 5/1965Reynolds.... 318/356 3,302,089 1/1967 Rosa et a1. 318/356 3,355,64011/1967 Lewis et a1..... 318/341 3,358,204 12/1967 Bradley 318/331 OTHERREFERENCES ELECTRONIC AND RADIO ENGINEERING by F. E. Term Pub. by.McGraw Hill 4th Edition 1955 Pages 721 and 723 (2 copies attached)Primary Examiner0ris L. Rader Assistant Examiner-K. L. Crosson Attorney-Wilson and Fraser ABSTRACT: A circuit for applying current pulsations ofopposed polarity to an inductive load wherein the net current in theload is the difference in the magnitude of the pulsations. A capacitanceis connected across the inductive load to permit the buildup of a netcurrent. Circulating currents in the capacitance can be restricted byinductance or resistance in series between the source of pulsations andthe capacitance.

PATENTEUJULHIH?! 3593077 sum 1 or 2 INVENTOR. RICHARD c. LOSHBOUGH a on:13S

EOHQMZ IOEQKMZUO ZIMEIPdE MN hm m N I CROSS-REFERENCES TO RELATEDAPPLICATIONS This application is a division of application SER. No.373,136 filed June 4, 1964 in the names of Robert E. Bell, Donivan L.Hall and Richard C. Loshbough, now U.S. Pat. No. 3,435,916 which issuedApr. 1, 1969 and is entitled Elevator Motor Speed Control Including HighGain Forward Loop and Lag-Lead Compensation." It is related to anapplication in the names of Donivan L. Hall and Richard C. Loshboughentitled Safety Circuits for Elevator Motor Speed Control IncludingNegative Feedback in a High Gain Closed Loop Ser. No. 758,776 filedSept. 10, 1968.

SUMMARY OF THE INVENTION This invention relates to electrical circuitsfor transmitting substantial amounts of power to inductive loads and isillustrated particularly by a controlled rectifier source producingelectrical pulsations of opposed polarity and of a controllablemagnitude for each polarity to apply a net current to a shunt field of adynamoelectric generator controlling a motor.

An object of the invention is to improve the efi'iciency of applicationof bidirectional pulsations of current to a highly inductive load todevelop a net current therein.

A more specific object is to avoid resistive losses due to high currentswhile permitting such currents to build in an inductive load.

A third object is to enable electrical pulses of opposite polarity andof controlled magnitude to be employed in the production of a net inputof a given polarity to an inductive load.

In accordancewith the above objects one feature of this inventionresides in a source of electrical energy supplying a highly inductiveload comprising oppositely connected controlled rectifiers havingcontrol gates arranged to place their respective rectifiers inconduction at the same point in the half cycle they are biased forwardlyand thereby impose a net zero energy when no input signal is applied andarranged to increase the phase angle for firing one rectifier whilereducing that for the other to produce useful net energy correspondingto the difference in the pulsations. A capacitance connected across theinductive load permits the net energy to buildup in the load.

Another feature, particularly applicable where higher levels of powerare required, involves an inductance connected in series with saidcapacitance between the sources of pulsating energy and the capacitanceto reduce the surge current in the capacitance.

A further feature resides in a controlled rectifier supply to thegenerator shunt field of a Ward Leonard system wherein capacitance isconnected across the highly inductive field to avoid the inductivelimiting effects on the rectifier supply. Excessive power dissipation asmight be experienced with a damping resistance is avoided by thecapacitance. The surge currents in the circuit are limited by includingan inductance in series with the capacitance across the inductive load.

DESCRIPTION OF THE DRAWINGS The above and additional objects andfeatures of this invention will be appreciated more fully from thefollowing detailed description when read with reference to theaccompanying drawings in which:

FIG. I is a diagram of a servocontrol including a block representationof a pattern generator. summing point, compcnsator and amplifier with aschematic ofa speed signal feedback circuit from a motor controlled in aWard Leonard system having a controlled rectifier supply to thegenerator shunt field and a controlled phase firing circuit for therectifier gates;

FIG. 2 through FIG. 10 are waveforms of the signals appearing at variouspoints in the firing circuit and output of the phase controlled,controlled rectifier source supplying the shunt field of the generatorsupplying the motor of FIG. I, the signals representing those presentwhen a zero input signal is applied to the circuit; and

FIGS. 2a through 10a are waveforms of the signals appearing at the samepoint as for FIGS. 2 through 10 respectively when a positive errorsignal is applied to the input of the circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention has beenapplied to elevator hoist motor controls wherein the armature 18 of thehoist motor is supplied with a variable voltage from a generatorarmature 21 driven by a motor (not shown), and the generator output iscontrolled by control of its shunt field EF. The hoist motor shunt field24 can be supplied from a suitable source (not shown) either at aconstant current level or levels controlled for torque or speedconsiderations in a manner well known in the elevator art. Controlledrectifiers SCRA and SCRB connected with opposite polarity supply thegenerator shunt field by applying thereto portions of an AC supply aspulses effective during the half cycle each is forward biased. A phasecontrol firing circuit is connected to the gates of SCRA and SCRB tocause greater conduction in one and less conduction in the other so thatthe controlled pulses of opposite polarity result in a not current ofthe dominant polarity in the load EF.

It is to be understood that other sources of pulses of opposed polarityand other loads can be utilized without departing from the spirit andscope of this invention and that the generator shunt field and the SCRsources are merely illustrative of the invention.

As shown in FIG. 1 a pattern generator 11, which can be a velocitypattern based on time for a hoist motor, provides the controllingsignal. The generator can produce a voltage scaled to velocity and of aform along the time axis which gradually increases from zero velocity,following a start signal, to a maximum acceleration, maintains aconstant maximum acceleration until it can make the transition tomaximum rated velocity smoothly and then gradually reduces theacceleration to zero at maximum velocity. The run of the elevator atmaximum velocity is dictated by a flat pattern signal from generator 11until slowdown is initiated. A gradual transition to a constant slopescaled to the maximum deceleration commanded of the elevator begins inthe pattern at slowdown and continues until zero velocity is approachedat which time a signal for the gradual transition between the maximumdeceleration and zero velocity is generated.

The pattern signal is combined at summing point 12 with a heist motorspeed signal derived from feedback lead 34 to produce a speed errorsignal representing the difference between the pattern speed commandedand the actual elevator or hoist motor speed. This error signal is thenfed through a compensating network, compensator 13, which adjusts boththe phase and magnitude of the signal to permit an increase in the gainand the bandwidth of the system within which it will operate withoutinstability.

A high gain buffer amplifier 25, one having a gain from 50 to I00,applies the compensated speed error signal to a phase control firingcircuit for the controlled rectifiers SCRA and SCRB to determine thevoltage level applied by the generator 21 to the armature 18 of thehoist motor.

In the illustrative example the amplified and compensated velocity errorsignal controls the phase of a firing circuit for a pair of controlledrectifiers connected with like polarity electrodes each connected to oneof the two terminals of a single phase alternating current source. Theserectifiers are triggered, for zero signal input to their firing circuit,by an alternating current shifted from the line phase so that they areeach conductive a like period and the net DC derived therefrom is zero.Changes in this supply to the load are achieved by raising or loweringthe base of the firing signal to increase the conduction interval in onerectifier over that in the other for a first polarity of pulsatingunidirectional current and to reserve that relationship for the oppositepolarity when the base of the signal is shifted to the other side of thezero signal level.

The speed signal is derived from a bridge arrangement as disclosed inRobert 0. Bradley U.S. Pat. application Ser. No. 368,623 which was filedMay 19, 1964 and is entitled Motor Speed Control" now U.S. Pat. No.3,358,204 which issued Dec. I2, 1967. This arrangement provides avoltage proportional to the e.m.f. generated in the motor, and thus themotor speed, while eliminating the effects of brush drop and armaturecurrent on that voltage. It involves providing pilot brushes 27 and 28on motor armature 18. A potentiometer 29 is connected across thegenerator interpole winding GIP, one main brush 30 of the motor and themotor armature 18 to pilot brush 28. A second potentiometer 31 isconnected from pilot brush 27 across main brush 30. With the taps 32 and33 of potentiometers 29 and 31 set so that the resistance of their upperportions is related to the resistance of their lower portions in thesame proportion as the external resistance provided by the generatorinterpole windings GIP is related to the motor armature resistance, thevoltage developed between taps 32 and 33 is proportional to the speedvoltage of the motor. In the example tap 33 is grounded and tap 32 isconnected through lead 34 to summing point 12.

The resulting error signal from summing point 12 is to fed tocompensator 13. The output of the compensator is connected by lead 57 tothe input of direct current amplifier 25. This amplifier may bestabilized by negative feedback with parameters providing the desiredamplifier gain.

The firing circuit of the controlled rectifiers is based upon adisplacement of the firing wave from the applied line wave so that apair of back to back rectifiers are fired symmetrically to produce nonet current at zero signal and are fired assymmetrically to apply eithera positive or negative net current on the generator shunt field EFdepending upon the direction of the shift in firing angle.

Transformers TI and T2 are each driven from the same line voltage sothat their inputs are in phase. The output of transformer T1 is phaseshifted 135 by the three, cascaded, phase shifting networks eachcomprising a condenser 81 and a re sistor 82. Exact adjustment of thisshift is obtained by means of potentiometer 83. This voltage is summedwith the output of the amplifier 25 in the summing network of resistors84 and 85. Resistor 86 connected from terminal 90 of a highly regulatedpositive source of direct current (not shown) to lead 87 and the base oftransistor Q offsets any threshold voltage of Q10.

In considering the firing circuit two sets of waveforms will beconsidered in FIGS. 2 through 10. The first set represents the signalsat various points in the circuit when zero input is applied at lead 57.The second represents the signals at corresponding points when apositive input or error signal exists. The second set will bedistinguished by a lower case a.

The waveform across the resistor 85, which is applied on lead 87 in aform modified by the clamping action of diode 88 and the base-emitterdiode of transistor 010 to the base of 010 with no output from amplifier25, is shown as a sine wave A shifted 135 from line sine wave B andhaving its origin shifted as shown in FIG. 2. The waveform at thecollector of 010 is shown in FIG. 3. Excessive reverse bias on 010 fromthe AC signal on 87 is avoided by the diode 88 which passes negativesignals above its threshold to ground. When the applied voltage reachesthe threshold voltage of 010, the transistor begins conducting currentand the drop in resistor 89 causes the collector voltage to drop atjunction 91. The collector wave form corresponds to the input until thetransistor becomes saturated and the curve flats.

Transistor O1! is an emitter-follower whose emitter voltage wouldcorrespond to the signal at junction 91 but for the clamping action ofthe base-emitter diode of transistor Q12. The dashed line in FIG. 3 isthe emitter waveform ofQlll.

The collector wave of 012 is shown in FIG. 5, and the waveform ofQll3 isshown in FIG. 4. Transistors Q12 and Q13 and their associated circuitryconstitute a Schmitt trigger wherein the triggering signal is developedat junction 91. When zero signal is present at 91, transistor Q13 isconductive and transistor 012 is held off.

As the base of 012 goes positive with the emitter of Q1], collector Q12draws current through resistors 92 and 93 reducing the voltage on baseQ13 below its sustaining level and terminating conduction in Q13 wherebyits collector voltage rises at junction 94. The increased voltage on thecontrol electrode of silicon controlled rectifier SCRA causes thatrectifier to conduct when its applied anode-cathode voltage fromtransformer T2 is in the forward direction. At this time the voltage atjunction 95 is the forward drop of diode 96 above ground and, in view ofthe forward drop of diodes 97, the voltage on the control electrode ofSCRB is brought to ground through resistor 98 to enable its conductionto be terminated.

When the base ofQl2 returns to ground, it is cut off and the voltage atthe collector of Q12 rises. This voltage is applied through the voltagedivider of resistors 99 and 1011, and diode 102 to the base ofQ13 sothat it initiates conduction. The voltage at junction 94 falls below thethreshold of diodes 103 so that the control electrode of SCRA isgrounded through resistor 104. At this time the potential at junction 95has risen so that when it exceeds the threshold of diodes 97 the controlelectrode of SCRB is driven positive beyond its threshold of conductionto enable SCRB to tire.

The collector signals of Q13 and 012 as shown in FIGS. 4 and 5 are atlevels V2 and VI determined by the conduction drop of the gates of SCRAand SCRB and the threshold voltages of the diodes 103 and 97 in thecollector circuits. The voltage in series with the SCRs and load is inphase with the line supplying the primary of T2. If the load wereresistive, the voltage across SCRB is shown in FIG. 6 while that acrossSCRA would be similar for the other half cycle. The resulting waveformacross a resistive load would appear as in FIG. 7.

The circuit for SCRA would extend from grounded junction 105 throughrectifier 106, junction 107, brake relay contact BK-S, resistor I08,relay contact BK- t, junction 109, closed power switch 111, thesecondary of transformer T2, fuses 112, switch 111, SCRA and junction105. Elevator brake relay BK (not shown) controls contacts BK-4 and BK-Sto connect the SCRs to field EF when the elevator is set to run and toestablish the suicide circuit through back contacts BK-I and BK-2 whenthe elevator is stopped and relay BK is deenergized. The correspondingcircuit for SCRB is traced through rectifier 113. It should be notedthat pilot lamp 114 is connected across the secondary of transformer T2to indicate power is applied to the firing circuit at terminals 115 and116 connected to T1 and to the SCR circuit. When the generator fieldpower is on, pilot lamp 117 is illuminate.

The true load on the SCRs is the highly inductive generator field EF andthe resistor 108 is significant only when the generator suicideconnections are made to permit the decay of the field. This inductiveload imposes limits upon pulsating current so that virtually no DC fluxcould be developed in the field winding alone. However, circulatingcurrents are permitted without any direct current loss by shunting fieldwinding EF with a large capacitance 118, e.g. I500 m.f. This arrangementis further enhanced in its operating characteristics, particularly withrespect to the surge currents through SCRA and SCRB, by including arelatively low inductance 119 in series with the capacitance as alimiting means, e.g. 0.01 henry and 0.16 ohm. This L-C series circuithas substantial advantage over a shunting resistor of low value in thatno DC power loss is incurred and the efficiency of the circuit isenhanced. Resistor 120 is ofa relatively high value, e.g. 1000 ohms, andtherefore passes negligible current to the applied signal. Its functionis to provide a discharge path for the L-C circuit when the power isdisconnected.

earlier and sustains that threshold This approach has been applied to a30K VA generator driving a motor in a system having a motor constant Kgof 43 volts/ampere, a generator shunt field resistance R, of 12 ohms, agenerator field inductance L, of 6 henrys, a loop inductance L of 0.0145henry, a loop resistance R of 0.175 ohm, and a motor constant K, of 30Newton-meters/ampere. The elevator car has a capacity of 3,000 pounds,an empty car weight of 6l00 pounds, and is counterweighted to 40 percentof rated capacity so that for full load operation the inertia .I is 922kilogram-meters 2/radian, and the torque is 3051 Newton-meters. The drumover which the hoisting cables are trained has a diameter of 30 inches.

As a result of the highly inductive load presented by field EF to theSCRs the current reaches its peak when the input voltage is zero. TheSCRs do not turn off until the current goes to zero even if theimpressed voltage has reversed sign. Therefore, the voltage across theinductive load of field EF is shown in FIG. 8. The voltage across SCRBfor this load is shown in FIG. 9. A corresponding voltage is developedacross SCRA for the other halfcycle under this load.

The filter composed of capacitance 118 and inductance 119 employed toovercome the high impedance presented to pulsating voltages by field EFand to smooth the SCR outputs has a current form as shown in FIG. 10.

Since the areas under the curves of FIG. 10 representing flow in eachdirection for SCRA and SCRB and in the filter are equal the net or DCvalue is zero and the generator shunt error field EF receives zero inputwhere the signal from amplifier 25 is zero.

A positive or negative signal from amplifier 25, indicating a velocityerror signal, as it appears at summing point 12 will alter the firingcircuit and produce a net DC input to the shunt field by shifting thephase of the firing signal. A positive signal indicative of a hoistmotor speed less than the speed commanded when the commanded speed isplus, increases the conduction interval of SCRB while decreasing theconduction interval of SCRA. This change tends to change the generatorvoltage in a manner to increase the motor speed and decrease the error.Conversely, a negative signal at summing point 12 for the same commandsignal will decrease the conduction interval of SCRB while increasingthat of SCRA. This will tend to retard the motor speed by reducing thecurrent in the field EF to reduce or reverse the impressed voltage onthe armature thereby decreasing the motor speed to tend to decrease theerror.

.If a positive voltage is present at summing point l2 the waveforms areas shown in FIGS. 2a through 10a. The firing circuit voltage waveform Aais shifted positively as shown in FIG. 2a with the result that itachieves the threshold of Q10 later to lengthen the interval ofconduction for SCRB as shown in FIG. 5a and shorten the interval forSCRA as shown in FIG. 4a. The resulting change in the voltage applied tothe field EF is shown in FIG. 8a. It will be noted that the flow in SCRBis substantially greater than in SCRA and a net current results causinga generator armature voltage which drives the motor 18. When the motorapproaches the desired speed, so that the speed voltage on lead 34balances the pattern voltage, the error signal approaches zero, thevoltage of amplifier 25 is zero and the net DC into the fields is zero.Any change in motor speed results in a speed error signal which forcesthe motor back to its proper speed.

It is to be understood that a source other than the pair of controlledrectifiers SCRA and SCRB can be employed to issue pulses ofa givenpolarity repetitively from one terminal, as at I07, and to issue pulsesof that polarity repetitively from a second terminal, as at 109. Thecontrol of these pulses is such that one dominates over the other toproduce a net current in the inductance. Inductive loads other thanshunt field windings for a generator can be supplied from the source ofpulses of opposite'polarity connected thereacross by shunting the loadwith a capacitance.

A range of magnitudes of circuit elements can be employed. While theshunting capacitance 118 has been illustrated as of 1,500 microfaradsfor the shunt field of a 30KVA generator supplying the armature of ahoist motor it should be recognized that the size of the capacitanceneed only be large enough to supply the current required of theinductive load during the intervals of nonconduction of pulses from thesource terminal issuing the dominant pulse. The current limiting means,choke 119, is also subject to a range of values determined by itsinductive reactance at the frequence of application of pulses from thesource relative to the capacitive reactance of capacitance 118 at thatfrequency. The choke adequately limits currents, particularly when thenet output is near zero and both pulses are of equal or nearly equal andyet substantial magnitude, when it is more than twice the reactance ofthe capacitance. The dissipation of instantaneous currents in theinductive-capacitive path 119 and 118, as when the load is disconnectedfrom the source, should be through a resistance 120 of sufficient valueto avoid excessive power loss in normal operation. Accordingly,variations in the combinations employed to illustrate this inventionfall within its spirit and scope.

I claim:

1. A hoist motor system for an elevator comprising a direct currenthoist motor; an armature for said motor; a shunt field for said motor;means for energizing said shunt field; a direct current generator; anarmature for said generator connected to said motor armature; a shuntfield for said generator; means for generating a signal proportional tothe speed of said motor; means for generating a speed command signal;means for combining said speed signal and said command signal to producean error signal; a source of alternating current having two terminals; apair of controlled rectifiers having like electrodes connected to theopposite terminals of said source, the electrodes of said rectifiersopposite said like electrodes being connected to a common point; acontrol electrode for each rectifier; means to periodically applysignals to said control electrodes of a magnitude and phase to initiateconduction in said rectifiers; means to control the magnitude of saidsignals to said control electrodes according to said error signal; saidrectifiers passing current to the shunt field of said generator asunidirectional pulses of opposite polarity; a capacitance connectedacross said generator field to enable said pulses to build up the directcurrent flux in said generator; and an inductance connected in serieswith said capacitance to limit the pulsating current drawn through saidrectifiers.

2. In combination a controlled source of electrical pulses, a firstterminal for said source repetitively issuing pulses of a first polarityof a controlled magnitude, a second terminal for said sourcerepetitively issuing pulses of said first polarity of a controlledmagnitude, an inductive load for said source, means connecting saidfirst and second terminals across said inductive load, a capacitance ofa magnitude to supply the current required of said load during theintervals of nonconduction of pulses from the terminal issuing thepulses of greatest magnitude connected across said inductive load, andcurrent limiting means in series with said capacitance across said load.

3. A combination according to claim 2 wherein said inductive load ofabout 6 henrys and said capacitance is of the order of 1,500microfarads.

4. A combination according to claim 2 wherein said current limitingmeans is an inductance.

5. A combination according to claim 6 wherein said inductance has aninductive reactance at the frequency of application of said pulsationsfrom said source of more than twice the capacitive reactance of saidcapacitance.

6. A combination according to claim 2 wherein said source comprisesfirst and second controlled rectifiers each having a main anode and amain cathode-electrode, said first and second terminals are like mainelectrodes of said first and second controlled rectifiers respectively,said load is a shunt field ofa dynamoelectric generator, and saidcurrent limiting means is an inductive reactance more than twice thecapacitive reactance of said capacitance at the frequency of applicationof pulses from said source, and said combination includes a resistanceacross said capacitance and said current limiting means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated Tuly 13 1971 Patent No. 3 593 077 Inventor(s) RICHARD C. LOSHBOUGH It is certifiedthat error appears in the above-identified patent and that. said LettersPatent are hereby corrected as shown below:

Column 3, line 30, "to fed" should be fed to Column 4, line 56,"illuminate should be illuminated Column 6, line 63, "6 should be 4Signed and sealed this 6th day of August 197A.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting; Officer Commissioner ofPatents

1. A hoist motor system for an elevator comprising a direct currenthoist motor; an armature for said motor; a shunt field for said motor;means for energizing said shunt field; a direct current generator; anarmature for said generator connected to said motor armature; a shuntfield for said generator; means for generating a signal proportional tothe speed of said motor; means for generating a speed command signal;means for combining said speed signal and said command signal to producean error signal; a source of alternating current having two terminals; apair of controlled rectifiers having like electrodes connected to theopposite terminals of said source, the electrodes of said rectifiersopposite said like electrodes being connected to a common point; acontrol electrode for each rectifier; means to periodically applysignals to said control electrodes of a magnitude and phase to initiateconduction in said rectifiers; means to control the magnitude of saidsignals to said control electrodes according to said error signal; saidrectifiers passing current to the shunt field of said generator asunidirectional pulses of opposite polarity; a capacitance connectedacross said generator field to enable said pulses to build up the directcurrent flux in said generator; and an inductance connected in serieswith said capacitance to limit the pulsating current drawn through saidrectifiers.
 2. In combination a controlled source of electrical pulses,a first terminal for said source repetitively issuing pulses of a firstpolarity of a controlled magnitude, a second terminal for said sourcerepetitively issuing pulses of said first polarity of a controlledmagnitude, an inductive load for said source, means connecting saidfirst and second terminals across said inductive load, a capacitance ofa magnitude to supply the current required of said load during theintervals of nonconduction of pulses from the terminal issuing thepulses of greatest magnitude connected across said inductive load, andcurrent limiting means in series with said capacitance across said load.3. A combination according to claim 2 wherein said inductive load ofabout 6 henrys and said capacitance is of the order of 1, 500microfarads.
 4. A combination according to claim 2 wherein said currentlimiting means is an inductance.
 5. A combination according to claim 6wherein said inductance has an inductive reactance at the frequency ofapplication of said pulsations from said source of more than twice thecapacitive reactance of said capacitance.
 6. A combination according toclaim 2 wherein said source comprises first and second controlledrectifiers each having a main anode and a main cathode-electrode, saidfirst and second terminals are like main electrodes of said first andsecond controlled rectifiers respectively, said load is a shunt field ofa dynamoelectric generator, and said current limiting means is aninductive reactance more than twice the capacitive reactance of saidcapacitance at the frequency of application of pulses from said source,and said combination includes a resistance across said capacitance andsaid current limiting means.